1. Field of the Invention
The present invention relates to an image processing device for synthesizing a plurality of images acquired by carrying out imaging a plurality of times under different exposure conditions so as to enlarge a dynamic range.
2. Description of the Related Art
In the field of FA (Factory Automation) and the like, a so-called visual sensor has been put to practical use as a device for optically testing defects in half-completed products in a manufacturing process, products before shipment, and the like, as well as for optically measuring the size thereof.
A measuring article needs to be appropriately imaged when performing tests and measurements using such a visual sensor. The dynamic range (tone range where light can be detected) of an imaging element such as a CCD (Coupled Charged Device) or a CIS (Complementary Metal Oxide Semiconductor Image Sensor) is generally finite. Thus, when the exposure time length upon imaging becomes relatively long, whiteout (halation) occurs entirely or partially, and luminance and color information on the measuring article cannot be accurately acquired. When the exposure time length upon imaging becomes relatively short, total underexposure occurs entirely or partially, and an effective luminance cannot be acquired. Furthermore, in the measuring article locally having a portion where reflectivity differs and the measuring article having a portion of a large curvature, whiteout and total underexposure may locally occur by the influence of lighting and the like.
Thus, if the difference between the minimum value and the maximum value of light intensity (light energy that enters per unit time) radiated from one measuring article exceeds the dynamic range of the imaging element, the entire measuring article cannot be appropriately imaged.
There is known a technique of enlarging the dynamic range by imaging one measuring article a plurality of times under different exposure conditions and synthesizing a plurality of images acquired therefrom. Such a process is also referred to as a high dynamic synthesizing process or a super latitude process (SL process). For instance, Japanese Laid-Open Patent Publication No. 2002-334326 discloses a visual sensor that enlarges the dynamic range with a small calculation processing load.
In the FA field, the measuring article to be the target of tests and measurements, is generally conveyed on a line of a conveyor or the like. Thus, if the same measuring article is imaged a plurality of times under different exposure conditions to enlarge the dynamic range as described above, the differences in relative position of the measuring article among the images acquired by the respective imaging processes due to the time length required for the series of imaging processes, the imaging view, and the conveying speed of the measuring article may not be ignorable. In other words, as long as the imaging processes by the imaging unit are performed in time-series, the measuring article that was positioned on the upstream of the conveyance path in the first imaging process moves towards the downstream of the conveyance path as the imaging processes proceed.
As a result, when the respective images are synthesized with the view of the imaging unit as a reference, an image in a shifted state appears. Such image shift may cause errors in tests and measurements.